The invention relates to transfer paper.
Transfer paper is used for printing textile and material provided with a polyester coating, in particular textile of polyester, and mixtures of polyester with other fibers. To this end, by means of common printing techniques (flexographic, offset, intaglio, or rotary screen printing), a pattern, design or printing image is applied to the paper. Depending on the printing technique, the ink is thin-fluid or in the form of a pasty mass. The ink or paste contains sublimable dye components. By means of heat, the sublimable components of the ink are subsequently transferred, in the transfer process, onto the surface that is eventually to be printed. In this process, the temperatures common for transferring the dyes are within the range of from about 170° C. to about 210° C. During the transfer of the dyes in the ink, by means of heat and pressure, a portion of the dyes often stays behind on the paper. The extent to which the sublimable dye is transferred from the paper onto the textile during the transfer process is referred to as transfer efficiency.
To reduce the amount of non-transferred dye in the transfer process, measures have in the past been proposed to improve the transfer ratio (transfer efficiency). One of these measures concerned the application of a layer (release or barrier layer) to the smooth side (i.e. the side to be printed) of the paper, causing the dye to be transferred onto the substrate more easily.
In the case of a barrier layer, this layer prevents the dyes of the ink from penetrating too deep into the paper. The layer may also be applied to the paper to ensure that the material that is applied to the layer can easily be given off again or removed otherwise. In this case, such layer is referred to as release layer. In many cases, the release and barrier function can be achieved by the same material.
Through the application of a release or barrier layer to the paper, less dye remains behind on the paper after the transfer printing process, which is economically advantageous. Suitable materials for this release or barrier layer are, in particular in the case of water-based inks, hydrophilic polymers such as carboxymethylcellulose. Through the application of the layer, the extent in which the dye is transferred from the paper onto the surface to be eventually printed is increased. The effect that the application of such layer has on the transfer efficiency of the dyes is, for instance, described in an article by Dr. U. Einsele and Prof. Dr. Herlinger, Melliand Textilberichte, 7, 1987, pp. 487-494.
As a matter of fact, applying such barrier layer to the reverse side of the paper (hence not the side to be printed) is known for preventing the “ghosting effect.” This effect occurs, inter alia, during the storage of the printed transfer paper. This storage is usually effected in rolls. Such paper with an anti-ghosting barrier generally has a porosity of about 200 ml/min.
The porosity is defined as the air permeability as determined according to the ISO standards. ISO standards applicable hereto are, inter alia, ISO standard 8791-2 for determining the roughness of the paper and ISO standard 5636-3 for the air permeability or porosity of the paper. This can be done with an L&W Bendtsen Tester of AB Lorentzen & Wettre, Kista, Sweden.
A drawback of the printing of paper, such as in this case transfer paper, by means of contact printing processes, and in particular a rotary screen printing process, is that a printing form has to be made, such as a screen or a template. The making of a printing form entails costs. These costs are as high for small batches (small lengths) or samplings as for great batches (great lengths). Consequently, for smaller lengths, samples and one-off designs, relatively high costs have to be made. For such utilizations, this generally renders the use of contact printing processes expensive.
Another possibility for the printing of transfer paper is the contactless printing process. In this process, a digital image is transferred onto the support material by means of an inkjet printer or another, for instance electrostatic technique. This technique has the advantage over the contact printing process that no templates, screens or other printing forms have to be used. When computer control (for instance DTP techniques) is used, it is possible to print an image directly onto the transfer paper.
Numerous publications are known relating to paper that is suitable for printing with an inkjet printer. Some of those will now be dealt with. For instance, European patent application EP-A 0 730 976 discloses a paper for an inkjet printer which is suitable for being printed with an ink based on a water-soluble dye which substantially contains carboxyl groups as hydrophilic functional groups, which paper does not contain calcium carbonate, while on at least the side of the paper that is to be printed, a water-absorbing pigment and an aqueous binder are provided as main components.
DE 19628342 describes a paper for inkjet printing provided with a synthetic layer which, after printing, can melt under the influence of heat to form a layer resistant to water and light.
DE 19604693 describes a paper for inkjet printing which comprises a layer containing pigment and binding agent, the pigment substantially consisting of bentonite and the binding agent consisting of a hydrophilic binder or a mixture thereof with a hydrophobic binder.
DE 19618607 describes a paper for an inkjet printer comprising a support material and a color-receiving layer, while on the color-receiving layer a layer is present built up from finely-porous cationic charge centers including inorganic pigments and/or fillers. The color-receiving layer may contain, inter alia, carboxymethylcellulose.
DE 19628341 describes a paper suitable for printing with an aqueous ink, in which a layer has been applied to a temporary support material, which layer consists of thermoplastic synthetic particles and a binder, while as binder, a carboxymethylcellulose can be used, inter alia.
EP 770729 describes a paper suitable for inkjet printing with water-based inks, in which dimensional instability is prevented by subjecting the paper, before the coating process, to a treatment which obviates the shrinkage caused by the coating process.
French patent specification 76022691 describes the composition of a water-based ink containing sublimable dyes, for printing transfer paper with an inkjet printer.
The inks for sublimation transfer printing that are used both in contact printing processes and in contactless printing processes can be water-based. Water-based inks are inks produced with water as main liquid component, in which the dye particles are dispersed in the liquid. To such inks, thickeners may be added to enable processing the ink as a pasty mass in, for instance, a rotary screen printing process. Inks as can be employed in the above-described processes typically contain dye particles having a particle size in the region around 0.1 μm.
A drawback of the use of water-based inks in a contactless printing process, in particular inkjet printing, is that the aqueous composition of the ink causes the different color areas to run into one another, so that a reduced color contrast is obtained. Consequently, as far as acutance of the image and contrast of the color areas are concerned, the result of the printing process is often of reduced quality. Also, the uniformity of the color areas may be adversely affected. This drawback of water-based ink occurs during the printing of known types of transfer paper by means of an inkjet printer. Paper types that are specifically suitable for inkjet printing are not suitable for the use as described hereinabove, either, inter alia because of an unduly low transfer efficiency.
Thickening the ink into a pasty mass, as in the contact printing process, does not apply to inkjet printing, because the ink can then no longer be jetted. With this, the problem concerning the flowing of the ink in the case of inkjet printing cannot be solved.
Hence, a dilemma is involved.
On the one hand, in a contact printing process, the flowing of the ink and the non-uniformity of the printed image can be prevented with a pasty ink, but this entails the higher costs of producing a printing form.
On the other, the costs of making a printing form can be avoided by a contactless printing process such as inkjet printing, but in that case, a thin-fluid ink is used and the ink can flow.